While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that S contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as hole transporting materials and/or electron transporting materials, that will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies and long lifetimes combined with high color purity.
A useful class of electron-transporting materials is that derived from metal chelated oxinoid compounds including chelates of oxine itself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline. Tris(8-quinolinolato)aluminum (III), also known as ALQ or ALQ3, and other metal and non-metal oxine chelates are well known in the art as electron-transporting materials. Baldo et al., in U.S. Pat. No. 6,097,147 and Hung et al., in U.S. Pat. No. 6,172,459 teach the use of an organic electron-transporting layer adjacent to the cathode so that when electrons are injected from the cathode into the electron-transporting layer, the electrons traverse both the electron-transporting layer and the light-emitting layer.
Diarylamine derivatives are the most common used materials for the hole transporting layer. However, the use of aromatic amines in close proximity to recombination zones where light is generated has certain disadvantages. Specifically, generation of amine radical-cations can cause quenching of excited states, thereby decreasing device efficiency. (see D. Y. Kondakov, J. Appl. Phys., 102, 114504 (2007)). Moreover, a C—N single bond is relatively weak (<105 kcal/mol) and can participate or initiate various OLED efficiency loss processes during operation. For a related example, see D. Y. Kondakov, W. C. Lenhart, and W. F. Nichols, J. Appl. Phys., 101, 024512 (2007).
Hydrocarbons such as anthracenes and other polycyclic aromatic hydrocarbons without amino substituents are also known as hole-transport materials. For example, the use of anthracenes as hole-transporting materials has been described in U.S. Pat. Nos. 6,465,115, 6,361,886, US2005/0233165, JP11-228951, U.S. Pat. No. 6,565,996 and US2007/0049778. Other types of non-amino substituted hydrocarbons suitable for hole transport layers have been disclosed in U.S. Pat. No. 6,596,415 and JP11-040356.
The use of non-amino substituted fluoranthenes in an electron-transporting layer has been described in US20060257684.
However, these devices do not have all desired EL characteristics in terms of high luminance and efficiency of the components in combination with low drive voltages.
Notwithstanding all these developments, there remains a need to improve efficiency while reducing or retaining drive voltages of OLED devices, as well as to provide embodiments with other improved features such as operational stability and color.